The invention relates to a pulse generator comprising a part, preferably a rotor which is movable with respect to a stator and means for generating a magnetic field which affects at least one Wiegand wire to generate in a coil pulses based on the Wiegand effect upon movement of the moving part.
Such a pulse generator is known from the German Offenlegungsschrift 26 54 755 in which the rotor is provided with a number of permanent magnets of alternating polarity and the stator is provided with a coil and a Wiegand wire. The latter is exposed to a magnetic field uniformly changing in size and direction when the rotor rotates. Because of the Wiegand effect, needle pulses are generated in the coil in an instantaneous manner when a given field strength is reached at the reversal of the magnetic field. These needle pulses are largely independent of speed and show an alternating positive and negative polarity.
In an article in the magazine "Electronics" of July 10, 1975 (pages 100 to 105), the Wiegand wire and the Wiegand effect are described in detail. The Wiegand wire consists of well known magnetizable materials, such as Permalloy and Vicalloy, which are alloys derived from nickel, iron, cobalt or vanadium. Such polycrystalline materials possess domains which can be oriented by applying a magnetic field so that after removing the magnetic field these materials themselves generate a magnetic field. If the material is then exposed to an opposite magnetic field, a demagnetization or reversed magnetization is caused when the coercive field strength is exceeded. Thus, the curve of magnetization shows the characteristic hysteresis. By special treatment of the wire, that is, by a twisting action, the core of the wire obtains a comparatively low coercive force while the shell is provided with a higher coercive force. The Wiegand effect is characterized by the fact that the demagnetization of the core is related to a very rapid reversal of the domains when the low coercive force is exceeded. Thus, a very short high-amplitude needle pulse can be generated in a coil. With a short Wiegand wire measuring about 1 cm in length and roughly 0.1 mm in diameter, a core coercive force of 10 Oe and a shell coercive force of 20 to 30 Oe have been measured.
In the described pulse generator, the Wiegand wire is exposed to a comparatively high magnetic field of alternating polarity because of the rotation of the rotor. Thus, positive and negative needle pulses are generated whereby the number per unit of time is a measure of speed. However, in addition to speed there are numerous applications in which the sense of direction is also of interest. This applies particularly to navigation systems in which both vehicle speed and sense of rotation of the wheels are measured by using a pulse generator connected to the wheels of the vehicle. Furthermore, positioning devices for machine tools require information on the direction of movement. Previously, it has been customary to determine the sense of rotation from the phasing of at least two pulses or pulse trains. Yet, this required a higher technical input for the pulse generators and for the circuitry to allow the determination of the phase sequence.